1. Field of the Invention
The present invention relates to a high purity dense silicon carbide sintered body, a reflection mirror made by polishing the silicon carbide sintered body and processes for making the same. The reflection mirror or a substrate thereof of the present invention is useful as a reflection mirror or a substrate thereof for X rays, laser beams, etc. and can be utilized in various optical devices and so forth.
2. Description of the Related Art
Reflection mirrors of X rays or laser beams are widely used and metal or glass substrates have been conventionally used for such reflection mirrors.
However, as a result of developments in technology, a high optical source output, high speed beam scanning and so on are required and good heat resistance and high elastic modulus are characteristics required of reflection mirrors.
Conventional metal substrates have a relatively rough surface and the reflection of a short wavelength ray is therefore low so that metal substrates cannot be utilized in a mirror for a light having a wavelength shorter than that of visible light or an electromagnetic wave. Further, since metal is not highly rigid but is dense, it is difficult for a metal substrate mirror to be made with precision and light in weight.
Glass substrates have poor thermal conduction and therefore cannot be used in connection with a high power beam since the glass will deform by the heat from a high power beam. Also, glass substrate mirrors cannot be made with precision because glass is not rigid.
In contrast, ceramic sintered bodies are desirable materials for the above mentioned requisites owing to good heat resistance and rigidity, but are difficult to apply to reflection mirrors because conventional ceramics have a high degree of surface roughness. Among other ceramic sintered bodies, silicon carbide sintered bodies are considered suitable for application to a reflection mirror as they are easily machined and they maintain stability when exposed to X rays, etc.
Silicon carbide, however, is difficult to sinter without sintering additives since silicon carbide is highly covalent. Accordingly, the following processes for sintering silicon carbide with sintering additives are known: The solid sintering method using the addition of both boron and carbon, as disclosed in Japanese Unexamined Patent Publication (Kokai No. 49-7311, published in 1974; and the liquid sintering method using the addition of aluminum oxide, as disclosed by R. A. Alliegro et al. in Journal of the American Ceramic Society, Vol. 39, P. 386, 1956.
Nevertheless, silicon carbide sintered bodies produced by the solid sintering method have insufficient density and cannot be utilized as a mirror substrate, which requires a density close to the theroretical density.
In contrast, it is said that the liquid sintering method may provide sintered bodies having a density close to the theoretical density. Silicon carbide sintered bodies obtained by the liquid sintering method are disclosed in, for example, the above Allingr's paper, Japanese Examined Patent Publication (Kokoku) Nos. 57-4153 and 60-34515 and Japanese Unexamined Patent Publication (Kokai) No. 63-21251.
Nevertheless, these silicon carbide bodies are intended to be used as a structural material and therefore contain aluminum compound particles due to the sintering additives or tungsten carbide particles, etc. due to the process in the sintered bodies, which cannot be used as a mirror substrate of a high purity dense silicon carbide sintered body.